An apparatus of this type is known from German Patent Specification No. 2,653,788. However, it has been found that the vacuum provided in the moulding chambers is insufficient to overcome the friction in the moulding material, which is compacted in the lower portion of the down pipe. The weight of the moulding material in the down pipe will be added to the compressive force provided by the intake force from the moulding chamber under vacuum, which gives rise to caking of comparatively moist moulding material in particular.